Bilic said: “It was a collective bad day. I told the players it was unacceptable and to make sure it was a one-off.

“You are going to lose games in this league but we weren’t good today, apart from in the first 10 minutes.

“We gave them the win. Watford were full of respect for us – but when they smelt that we weren’t penetrating enough, and that our movement and passes were slow and backwards, they smelt the opportunity to go at us.

“We learned a lesson, and I see no reason why we won’t come back and play like we did before.”

The Hammers, without the injured Diafra Sakho, handed a first start since January to fit-again Andy Carroll.

But unfortunately for them, the big frontman was more involved in his own penalty area than the opposition’s – with some calamitous results.

Carroll was West Ham’s hero from the bench against Chelsea last week, but he got himself in an almighty mess from Ben Watson’s free-kick, allowing Nathan Ake to drill in a cross which was turned in by Ighalo and defender Aaron Cresswell.

If there was any doubt about who got the final touch on that one, there was none just after the break when Ighalo collected Ikechi Anya’s square ball and crashed an unstoppable drive into the top corner from 15 yards.

West Ham’s misery was complete when, with four minutes to go, James Collins clattered into the back of Ighalo on the halfway line and was shown a straight red card.